The 81-year-old Supreme Court justice should retire this summer to ensure Obama can choose a like-minded successor.

Justice Ruth Bader Ginsburg should retire from the Supreme Court after the completion of the current term in June. She turned 81 on Saturday and by all accounts she is healthy and physically and mentally able to continue. But only by resigning this summer can she ensure that a Democratic president will be able to choose a successor who shares her views and values.

A great deal turns on who picks Ginsburg's successor. There are, for example, four likely votes to overturn Roe vs. Wade on the current court: Chief Justice John G. Roberts Jr. and Justices Antonin Scalia, Clarence Thomas, and Samuel A. Alito Jr. If a Republican president selects Ginsburg's replacement, that justice easily could be the fifth vote needed to allow the government to prohibit all abortions. On many cases — including ones involving environmental law, healthcare, gay marriage, the death penalty and the rights of those in Guantanamo — the four liberal justices have joined with Justice Anthony M. Kennedy for a progressive result.

But if a conservative had been occupying Ginsburg's seat when the court heard those cases, the rulings might well have been very different, and if a conservative takes her seat when she leaves, they might not survive.

There likely will be many calls, publicly and privately, for Justice Ginsburg to resign before President Obama leaves the White House to prevent the risk of a Republican being able to appoint her successor. But simply leaving before the next election isn't enough. If Ginsburg waits until 2016 to announce her retirement, there is a real chance that Republicans would delay the confirmation process to block an outgoing president from being able to fill a vacancy on the Supreme Court. In fact, the process for confirming nominees for judicial vacancies usually largely shuts down the summer before a presidential election.

Moreover, there is a distinct possibility that Democrats will not keep the Senate in the November 2014 elections. The current Senate has 53 Democrats, two independents who vote with the Democrats and 45 Republicans. But in the November 2014 elections, Republicans have a far greater likelihood of gaining seats in the Senate than the Democrats. One recent study identified nine seats held by Democrats that could be won by Republicans, but only two seats occupied by Republicans that might be taken by Democrats.

So long as the Democrats control the Senate, President Obama can have virtually anyone he wants confirmed for the Supreme Court. There has been only one filibuster against a Supreme Court nominee, and that was to block Justice Abe Fortas' elevation to chief justice, not to block his initial appointment. There were 48 votes against Thomas and 42 against Alito, but Democrats filibustered neither. Besides, if Democrats have control of the Senate, they could change the rules to eliminate the filibuster for Supreme Court nominees, just as they did for lower federal court judges and presidential appointments to executive positions.

In the end, the only way to ensure that President Obama can pick someone who will carry on in Justice Ginsburg's tradition is for the vacancy to occur this summer. Indeed, Justice Stephen G. Breyer, who will turn 76 this summer, should also carefully consider the possibility of stepping down this year.

Some might question whether a justice should be so calculating in choosing when to retire. But not doing so ignores the reality that ideology matters enormously in Supreme Court decision-making. This is nothing new; ideology always has mattered, and which president fills vacancies on the court can have an impact for decades. If, for example, Al Gore or John Kerry rather than President George W. Bush had replaced William Rehnquist and Sandra Day O'Connor in 2005, a liberal majority would likely have endured on the court for the next few decades. Conversely, if John McCain had replaced David Souter and John Paul Stevens, there would be a solid conservative majority and Roe vs. Wade surely would have been overruled already.

I do not minimize how hard it will be for Justice Ginsburg to step down from a job that she loves and has done so well since 1993. But the best way for her to advance all the things she has spent her life working for is to ensure that a Democratic president picks her successor. The way to facilitate that is for her to resign this summer.